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Solar Interior

The Central Core

The central core extends out to 0.25 solar radii. It is where all the energy for solar activity is created. Despite the very high pressures due to gravity pulling all the mass inwards, the intensely high temperatures (15 million Kelvin) ensure that the matter is kept in a fully ionised gaseous state (the Hydrogen molecules are broken down into Hydrogen atoms and have their electron stripped away). This leaves Hydrogen nuclei, a proton, available for thermo-nuclear reactions. As a result of this, the principle means of energy production is the Proton-Proton chain. Basically, the Hydrogen nuclei fuse together in a series of reactions to form Helium nuclei and energy.

It does this in three stages:

1) Two protons (Hydrogen nuclei) fuse together to create Deuterium (proton and neutron) with a positron and a neutrino also formed in this process. This is done when the two protons come within 10^-15 m of each other and simultaneously decay into a neutron and a positron.

2) This Deuterium nucleus fuses with another proton to form a very unstable isotope of 3He (two protons and a neutron) and a gamma-ray photon.

3) This 3He isotope now has to fuse with another 3He isotope to for the required 4He nuclei and two extra protons. This is quite a difficult task as the 3He nucleus has a very short half life, as it is so unstable. About 600 million tons of the Sun's Hydrogen is converted into Helium every second, releasing about 5 million tons of energy.

The Radiative Zone

Energy is primarily released in the central core in the form of gamma-ray photons, as said before. They start to make their way out to the surface. Between 0.25 and 0.7 solar radii, it does this via radiative diffusion. The photons start to make their way out of the core in a series of `random walks'. They don't travel very far before colliding and being absorbed by other gas molecules. They are absorbed and photons of the same wavelength re-emitted many times. They are converted from gamma-ray / x-ray photons to Ultraviolet photons and then finally to visible photons when they reach the photosphere. As a result of these random walks, the average time taken for a photon to leave the surface is between 10^4 - 10^5 years.

The Convection Zone

Beyond 0.7 solar radii, convection currents dominate the transport of energy to the surface. Hot gas rises upwards and then cooler gas replaces it. This is because the density of nuclei is lower than in the radiative zone, so radiative diffusion cannot take place. Energy is transported faster here than in the radiative zone, however it still takes a very long time for a photon to reach the surface, as collisions still occur.